The present invention relates to semiconductor lasers with stable output characteristics useful for optical signal transmission over optical fibers.
As a semiconductor laser ages, it requires more drive current to output the same optical power level. The output frequency tends to drift away from a desired output frequency because of this necessity of increasing the drive current to maintain the output power as the laser ages. In applications such as telecommunications, where the laser""s operating optical frequency must remain within the narrow range of a specified channel, the age-related frequency drift is a problem. Although the input driving current could be kept relatively constant as the laser ages in order to minimize the frequency drift, the output power will then decline over time, which is also detrimental in fiberoptic communications.
Others have attempted to solve this problem by the use of negative feedback systems that actively adjust the laser chip temperature. That is, as the drive current is increased due to laser chip aging, a detection system compensates the temperature of the laser chip to overcome the frequency shifting effects of the increasing drive current. However, monitoring the drive current does not allow effective frequency control, and increasing the chip temperature degrades its lifetime. Hence, this prior solution is not completely adequate.
An object of the present invention is to provide a mounted semiconductor laser device, and a method of constructing the same, that has an age-stable output frequency.
The optical frequency at which a laser operates can be maintained within a more narrow range over the lifetime of the laser by providing a specified thermal impedance between the active region of the laser and the heat sink.
Specifically, two competing mechanisms have opposite effects on the index of refraction in the laser waveguide. First, the increased drive current needed to maintain laser output power levels as the laser ages has the effect of increasing the carrier density in the active region of the laser. The increased carrier density (i.e., electrons and holes) lowers the index of refraction of the material. Second, the increased current causes the laser to be heated. The increased temperature raises the index of refraction. Depending on the way the laser is mounted to the heat sink the thermal impedance may be relatively large or small, changing the size of the temperature swing so that one or the other of these mechanisms dominate. If the laser is mounted with the active region down, i.e. in close proximity to the heat sink with sufficiently low thermal impedance, the carrier density effect dominates and the index of refraction decreases over the lifetime of the laser. If the laser is mounted with the active region up or with sufficiently high thermal impedance, the thermal effects dominate and the refractive index increases over the lifetime of the laser.
In this invention, the thermal impedance is chosen to have a value that results in substantially complete compensation between the two effects. Basically, because the increased current density and increased temperature drive the refractive index of the laser active region in opposite directions, the effects can be essentially cancelled by an appropriate choice of heat sink thermal impedance. The laser may be mounted active region down, but the thermal impedance is selected so that neither effect dominates. This maintains the refractive index of the active region of the laser within a more narrow range, which in turn maintains the operating frequency of the laser at a more accurately controlled optical frequency (or output wavelength).
In order to provide the required amount of thermal impedance, a layer of material having lower thermal conductivity than the heat sink (such as SiO2) is formed between the active layer and the heat sink. It may be formed either on the laser chip or on the heat sink, or on any element interposed therebetween. In a preferred embodiment, the impedance for the required thermal effect can be calculated from measurements of lasers subjected to accelerated aging. Then the calculated thermal impedance is embodied in the silica layer upon fabrication, either by selecting the thickness of the layer, or by selective changes to the composition (e.g. by doping) of the silica layer to adjust its thermal conductivity to the desired level, or both. Essentially, the thickness and composition of the thermal impedance layer will be pre-chosen to obtain a constant index of refraction in the laser despite aging of the device.
An advantage of the invention is that we can obtain a substantially constant index of refraction effect when the drive current is increased as the laser diode ages. With this very much flatter index of refraction change versus laser age, the frequency of the output light generated by the laser would stay longer within design limits. Adjustments would be needed less often, with the laser lasting much longer in service due to its longer operation within the design parameters.